1. Field of the Invention
This invention relates generally to the design of sensors for detecting structure borne sound, and more particularly, to sensing devices for detecting sound generated by persons trapped in collapsed structures.
2. Discussion of the Prior Art
Thousands of people are killed or injured every year in building collapses caused by earthquakes, explosions, or other disasters. Immediately after a disaster, it is important to search the collapsed structures for live victims. A common method of locating live victims is to listen for movement or tapping sounds. This invention relates to the design of electrically amplified listening devices for detecting structure borne vibrations.
In an electrically amplified listening device, an acoustic/seismic sensor is attached to the rubble pile. The sensor consists of a housing that protects the speakers or headphones or displayed on a meter, barograph or chart recorder. A search specialist listens to the generated sounds and reviews the display and determines if a victim is present.
The usefulness and accuracy of the listening device is determined to a large extent by the design of the sensor and the transducer used. Transducers can measure position, velocity or acceleration of the structure and these transducers are sensitive only along a single axis. When the direction of the vibration is not in line with the axis of the transducer, the signal is attenuated. If the direction of the vibration is orthogonal to the transducer axis, the vibration will not be picked up at all. Therefore, it would be desirable to have a transducer which is a sensitive long multiple axis.
One solution to this problem is to have multiple transducers each aligned along a different axis in the sensor. Although this is possible, multiple transducers increase the size and expense of the sensor. A better solution is to have a single axis transducer that can pick up vibrations along multiple axes, and this is the object of this invention.
Another difficulty in locating trapped victims is the uneven nature of the rubble pile. The rubble can be oriented at random angles and the searcher should be free to attach to the sensor the rubble in the most convenient fashion. This requires that the sensor be able to function at a variety of angles and orientations and have multiple surfaces that can couple to the rubble. Furthermore, the amplitude of the detected signal should be independent of the specific orientation of the sensor relative to the rubble pile.
This invention relies upon the fact that rectangular plates and beams are the most common building materials used. Vibrations in these plates or beams are usually aligned along one axis, sometimes aligned along two axes, but very rarely aligned along all three axes. This invention uses a cube-shaped housing with the transducer placed diagonally between two opposite corners. This ensures that the transducer is at an angle of approximately 45 degrees relative to the surface of the plate or beam and the detected signal will be invariant to which side of the housing is placed on the surface. Such a sensor is guaranteed to pick up all vibrations that are aligned along a single axis of the plate or beam.